The long term objective of this project is to develop an understanding of lifestyle factors that influence the cognitive and brain health of children while also reducing the sedentary nature of today's youth. Previous research conducted by our research team and others has found that physical activity interventions can enhance both a variety of aspects of cognition and brain structure and function of children, older adults, and individuals with neurodegenerative disorders such as Parkinson's disease and multiple sclerosis. More specifically, in our research with children we have found that higher fit children possess larger hippocampi which in turn are related to better relational memory than their lower fit counterparts. We have also observed that higher fit children exhibit more efficient executive control as indicated by performance measures and event-related brain potentials. While intriguing, these cross-sectional data do not enable us to establish causality between physical activity and cognition. In the current proposal we substantially extend this previous research by examining the influence of a 9 month randomized controlled afterschool physical activity program on cognition and brain health. Cognition will be assessed with a battery of tasks and standardized achievement tests both before and after the 9 month intervention in the activity group and a wait list control (who will receive the intervention the following year). Children will also participate in magnetic resonance imaging (MRI) sessions both before and after the intervention (and at comparable times for the wait list control). In these sessions we will measure both structural aspects of the brain including regional volumes of gray matter and the integrity of the white matter tracts (through diffusion tensor imaging) and functional aspects of brain function using fMRI activity recorded as the children perform a series of executive control and memory tasks. We anticipate, based on our cross-sectional studies with children and our previous longitudinal studies with older adults, that the children in the physical activity program will show both larger regional brain volumes, particularly in brain regions that subserve executive control and relational memory, and more efficient brain function, as indexed by task-related and resting state fMRI. Furthermore, we anticipate that these changes will be accompanied by improvements in memory and executive control processes. Given recent trends identifying decreased levels of physical activity and health status in preadolescents, the understanding of the potential benefits of physical activity on cognition is of great interest. It s imperative that factors positively influencing cognitive function of children be examined to maximize health and effective functioning of individuals as they progress through the lifespan.